Paper products such as paper towels, napkins and toilet tissues are widely used on a daily basis for a variety household and commercial needs. Typically, such products are formed of a fibrous elongated web which is either packaged in rolls or a folded stack. The fibrous webs are embossed to increase the bulk of the tissue and to improve the absorbency, softness and appearance of the product. Embossing can also aid in holding adjacent plies of the web together. Additionally, embossing may be carried out in a particular pattern which designates an origin of the paper product or a commercial entity which utilizes the paper product. Generally, the embossing apparatus will include one or more rolls having protuberances and/or depressions formed therein for forming the embossed pattern and generally a corresponding backup roll which presses the web against the embossing roll such that the embossed pattern is imparted to the web as it passes between the nip formed between the embossing roll and the backup roll.
In fiber-to-steel embossing operations, a fiber roll is utilized as a backup roll with the fiber roll formed of a hard cloth-like material. The embossing roll is formed of steel and includes the protuberances and/or depressions engraved therein. Prior to use of the rolls for embossing, the embossing roll and backup roll are run together without a web passing therebetween with soap and water utilized for lubricating and softening purposes. The embossing roll and backup roll would be run together until the fiber backup roll took on a pattern corresponding to the protuberances and/or depressions of the embossing roll. The use of the rolls in embossing of paper products did not begin until after a pattern corresponding to the embossing roll was achieved in the backup roll. Generally, this would require 24 to 36 hours of operation, and thus the fiber roll approach required a great deal of initial start up time and costs associated with operating the rolls without embossing web products. Moreover, the steel rolls utilized in conjunction with this process are expensive to manufacture and thus interchangeability of such rolls is not practical.
In a later approach, steel-to-steel embossing rolls were used wherein protuberances and/or depressions are engraved on a roll and corresponding protuberances and/or depressions are engraved in a backup roll. As the web is passed through the nip formed between the rolls, the protuberances and/or depressions emboss the web and are accompanied by the protuberances and/or depressions in the backup roll. To prevent damage as a result of interference between the corresponding protuberances and/or depressions, a clearance of 0.003 to 0.007 inches must be provided. Due to the required clearance, the steel to steel approach was not as successful in softening the fibrous product since the clearance reduces the breaking of the fibers or fiber bonds as compared to other approaches in which the web is softened by working the web, that is by fracturing fibers or fiber bonds in the web. Moreover, as with the previous system, engraved steel rolls are expensive to manufacture and thus interchangeability of such rolls is generally not a viable option.
In rubber-to-steel embossing, the steel embossing roll is provided with protuberances and/or depressions and the web is pressed against the embossing roll by a rubber backup roll as the web passes through the nip formed between such rolls. The rubber backup roll accommodates the protuberances and/or depressions by virtue of its resilience and the rubber flows about the protuberances and/or depressions as force is applied to urge the rolls together. However, to ensure that the rubber flows about the protuberances and/or depressions to achieve an acceptable embossed pattern, an extremely large amount of force is required which in turn can increase production costs. In an attempt to overcome the aforementioned shortcomings, a rubber-to-steel mated embossing roll as set forth in U.S. Pat. No. 5,269,983 assigned to the assignee of the present invention, the contents of which are hereby incorporated herein by reference, was developed which mates a steel embossing roll having a plurality of protuberances extending therefrom with a rubber backup roll which urges the fibrous web substrate against the embossing roll thereby imparting a highly defined embossed pattern to the paper substrate for forming paper towels, napkins or tissues. As the paper substrate passes through the nip between the rolls, the web is forced about the protuberances and against the land areas of the steel roll, as well as into the indentations and outer peripheral surfaces of the rubber roll. As a result, a highly defined embossed pattern is provided. This is accomplished by laser engraving the rubber backup roll in order to provide mated indentations corresponding to the protuberances of the embossing roll. Due to the indentations in the rubber roll, significantly less pressure is required between the embossing and back-up rolls for causing the rubber to press the web against the protuberances and against the land areas of the embossing roll. Thus, the problems associated with wear, particularly heat-related wear of the prior art rubber to steel embossing devices is avoided. Additionally, since a large amount of force or pressure is not required for forcing the rubber to flow about the protuberances, problems associated with non-uniform or insufficient force along the length are avoided such that a more consistent pattern is imparted to the web along the length of the roll while reducing costs associated with the operation of the system. However, the aforementioned system still requires a costly engraved steel embossing roll. Consequently, frequently changing the pattern from one embossed pattern to a different embossed pattern remains cost prohibitive, in that high fixed and variable costs combined with long delivery times are typical for initially manufacturing tooling and masters for each unique embossing pattern which are subsequently employed in the chemi-mechanical engraving process to produce each embossing roll.
While steel embossing rolls can be recycled, i.e. the embossing pattern can be removed from the roll and a subsequent pattern reengraved thereon, eventually the wall thickness of the steel roll will become thinned resulting in an unusable roll. That is, conventional steel embossing rolls typically include a cylindrical wall thickness of approximately three inches. Consequently, over time the wall thickness will be reduced to a point where the roll is no longer usable, this being at a wall thickness of approximately 11/2 inch. Accordingly, having a core which is reusable indefinitely over time would result in a significant cost savings.
As noted from U.S. Pat. No. 4,144,813 mandrels having printing sleeves positioned thereon have been in use for quite some time in printing applications. These sleeves are generally formed of fiber reinforced resin or nickel alloys having a synthetic rubber coating or removable thin rubber plate affixed thereon with the outer surface being engraved or otherwise prepared for printing. However, this engraving is carried out merely to form a printing pattern wherein it is only the top surface of the pattern which is critical. Unlike embossing patterns, the side walls and contour of the printing elements are not critical to the performance of the printing operation, in fact, printing elements having curvalinear side walls and spherical surfaces would be undesirable and a detriment to the printing process. Further, with printing processes, the printing roll merely lightly contacts the sheet being printed and the fibers of the material being printed are preferably not damaged. However, with embossing processes, the embossing elements press into the material intentionally breaking and working the fiber bonds of the material so as to provide a strong but absorbent sheet having a soft texture and aesthetic appearance.
Accordingly, there is clearly a need for an embossing system wherein the embossed pattern may be routinely changed at minimal expense as desired. Such changes may be required as seasonal merchandise, corporate merchandise or personalization or product attribute improvements are desired. This need is satisfied in accordance with the present invention by way of an embossing system including rolls having interchangeable sleeves so as to allow the embossing pattern carried out by the system to be readily and routinely changed.